This invention relates to headers for heat exchangers, and more particularly, to headers and heat exchangers incorporating such headers which are designed for extremely high pressure applications.
Concern for global warming and the deterioration of the ozone layer as a result of the escape of fluorine containing refrigerants from refrigeration systems, including air conditioning systems, has prompted a new look at refrigeration systems utilizing more environmentally friendly refrigerants. One such system under study is a carbon dioxide (CO2) based system wherein CO2 is employed as the refrigerant. CO2 systems operate at significantly higher internal pressure than do conventional systems employing fluorine based refrigerants and as a consequence, there is a need to improve the pressure resistance of heat exchangers used in such applications as, for example, the gas cooler and the evaporator of such systems.
At the same time, these systems have the potential for extensive use in vehicular air conditioning systems where weight, because of its impact on fuel economy, is of considerable concern. This consideration makes it impossible to achieve the desired pressure resistance simply by expanding wall thickness of conventional heat exchangers used in such systems without other major changes because of the added weight of thicker walled elements. Furthermore, this solution is not an economically viable one because taking existing components without changing their size other than to increase wall thickness to achieve pressure resistance means more material will have to go into the heat exchanger, most notably in the headers, thereby increasing the cost of the resulting heat exchanger.
Various solutions to this problem have been proposed. For example, many of the heat exchangers employ tubular headers which are generally cylindrical in shape. Conventional flattened tubes have their ends fitted in tube slots in the headers, which tube slots are transverse to the direction of elongation of the header. It has been proposed to reduce the diameter of the header and reorient the tube slots so that they are elongated in the direction of elongation of the header. The tubes are then provided with a twist near where their ends enter the header so as to present a desired orientation of the tubes for air flow between the tubes through the heat exchanger.
One primary difficulty in this approach is that with smaller diameter headers, the process of forming the tube slots in the headers has become increasingly difficult. In order to have a desired wall thickness in the smaller diameter headers, it has been necessary to form the tube slots by machining procedures as, for example, by milling. Unfortunately, these machining operations are time consuming and expensive and are particularly more costly than the various punching techniques that have been used to form transverse tube slots in cylindrical headers in conventional heat exchangers utilizing conventional refrigerants.
Thus, there is a real need for a less costly header for use in high pressure heat exchangers, such as those used as condensers, gas coolers and/or evaporators in high pressure refrigeration systems. The present invention is directed to meeting that need.
It is the principal object of the invention to provide a) a new and improved method for making a header for a high pressure heat exchanger, b) a new and improved header with high pressure resistance for use in high pressure heat exchangers, and c) a new and improved heat exchanger having improved pressure resistance enabling it to function in a high pressure system as, for example, a high pressure refrigeration system such as a CO2 refrigeration system.
According to one facet of the invention, there is provided a method of making a high pressure resistant header for a heat exchanger which includes the steps of a) providing an elongated header structure including a central cylindrical passage surrounded by a wall of sufficient thickness to resist deformation when a fluid is placed within the passage at an operating pressure at which deformation is to be resisted, b) thinning the wall along its length by providing a first mating surface on a part thereof so that the wall, at the first mating surface is sufficiently thin that tube slots may be formed therein by punching as opposed to more expensive machining procedures, c) punching tube slots at predetermined spaced intervals of the wall at the first mating surface, d) providing an elongated strip having a second mating surface complimentary to the first mating surface and of a thickness such that the combined thickness of the strip and the wall at its first mating surface is about equal to or greater than the desired thickness of the wall, e) punching tube slots in the strip at the predetermined spaced intervals which are of substantially the same size and shape as the tube slots in the first mating surface, f) abutting the second mating surface of the strip to the first mating surface of the header structure with the tube slots in each being aligned with one another and g) thereafter bonding the strip to the header structure along their respective lengths to provide a unitary header with tube slots therein.
In a preferred embodiment, both of the mating surfaces are flat surfaces.
A preferred embodiment also contemplates that steps a) and b) are performed simultaneously by extrusion of the header structure.
Preferably, the first mating surface is formed on the exterior of the header structure.
In one embodiment, step b) is performed by providing a strip receiving groove in that part of the header structure exterior surface and the groove has a flat bottom surface defining the first mating surface.
In one embodiment, the header structure has a semi-oval exterior surface with the first mating surface being located between the sides of the semi-oval.
In one embodiment, the first mating surface extends between the sides of the semi-oval.
According to another aspect of the invention, a header for a high pressure heat exchanger is provided. The header includes an elongated tubular like element having a central, generally cylindrical passage in a tube receiving side. The element is a unitary structure and has a relatively thick wall partially surrounding the passage and a relatively thin wall at the tube receiving side. A first mating surface defined by a relief is located at the relatively thin wall of the element and a plurality of punched first tube receiving slots are located at the first mating surface and are in fluid communication with the passage and are located at predetermined spaced intervals. An elongated strip having a second mating surface complementary to and abutted against the first mating surface is provided such that the thickness of the strip and the thin wall is substantially equal to or greater than the thickness of the thick wall. A second plurality of tube receiving slots are located in the strip and are punched therein and located at the same predetermined intervals as the tube slots in the first mating surface and are of generally the same size and shape as well. They are aligned with the first tube receiving slots. A joint is provided that bonds the element and the strip together.
Preferably, the joint is a brazed joint.
According to still another facet of the invention, a high pressure heat exchanger is provided and includes a header as described previously. The tube slots are elongated in the direction of elongation of the element forming the header and a plurality of tubes, each of flattened cross section are provided and have their ends disposed within corresponding ones of the tube slots. The ends of the tubes are twisted about 90xc2x0 to the remainder of the corresponding tube and fins extend between and are bonded to adjacent ones of the remainders of the tubes.
Preferably, the fins are serpentine fins.
Other objects and advantages will become apparent from the following specification taken in connection with the accompanying drawings.